1. Field of the Invention
The present invention relates to the cutting of materials with laser radiation and more particularly to means for supplying gas to the kerf in a laser system operating with a high power density.
2. Description of the Prior Art
In the metal working field various techniques have been developed for cutting materials with laser radiation. The interaction between a beam of laser radiation and a material such as a metal is basically very similar whether welding or cutting is the desired result, however, cutting is encouraged by the appropriate location of flowing gas to the reaction region of the workpiece. Various types of gases are used in cutting, some being inert and others being very reactive with the workpiece per se in an exothermic fashion to actually enhance the cutting action of the laser radiation. The flowing of a gas into the fissure during a cut serves several purposes, the principal ones of which are the removal of molten material which is formed in the cut and the minimization of contamination of the laser beam focusing optics by material emitted from the reaction region and other foreign elements in the area of the weld.
One of the early efforts in this area is taught by Sullivan et al. in U.S. Pat. No. 3,597,578 entitled Thermal Cutting Apparatus And Method filed on Mar. 13, 1968. Sullivan et al. discuss cutting with a gas laser with a jet of gas directed into the kerf along the centerline axis of propagation of the laser beam. The gas reacts with the workpiece and enhances the cutting characteristics of the system. One of the shortcomings of Sullivan et al. is its power handling limitation. The optical train which brings the laser radiation to the workpiece from a source includes a chamber positioned adjacent to the workpiece and a focusing lens. The chamber serves to direct gas onto the workpiece in the desired direction and the lens forms a window in the path of the laser beam. The materials currently available to transmit laser radiation, particularly at infrared wavelengths, limit the amount of power which can be delivered to the workpiece. These materials also impose a restriction on the pressure at which the chamber which contains the cutting assist gas is operable. In many practical applications, the power density and gas pressure required for optimum cutting often exceed the limits imposed by transmitting window technology.
A related teaching is provided by Schlafli in U.S. Pat. No. 3,601,576 entitled Method For Boring Workpieces By Laser Pulses filed on Sept. 13, 1968. Schlafli discloses the use of air and other gases to scavenge the bore and clear away ejected residue primarily from workpieces which are drilled by pulses of laser radiation. A low pressure region provided below the workpiece to assist in the removal of materials from the work area is also disclosed. Schlafli teaches the use of gases which are both on axis and off axis, and the technique is governed by the restraint that radiation transmissive optics are required to contain and control the gas flow to the workpiece.
In U.S. Pat. No. 3,629,546 entitled Air Cooled Laser Processing Of Materials filed on Apr. 2, 1969, Frye also discloses the use of a gas to assist in cutting. The Frye invention includes the same major elements of the prior art discussed above, namely, a source of laser radiation, a gas jet, and a workpiece to remove the vaporized material from the region. Frye is not concerned with keeping the gas flow on the axis of propagation of the laser radiation since he is using the gas only to remove vaporized material from the region immediately above the workpiece.
A related teaching is provided by Houldcroft in U.S. Pat. No. 3,679,863 entitled Thermal Cutting Apparatus filed Nov. 10, 1969. Houldcroft is similar to the materials discussed above and contains the additional recognition that an atomized stream of liquid coolant can be directed onto the workpiece in the vicinity of the cut to prevent charring and burning of various types of materials. In U.S. Pat. No. 3,612,814 entitled Cutting Process Employing A Laser filed Jan. 6, 1970, Houldcroft teaches still another system which deals with the controlled flow of gas to the cut region to affect the speed and quality of cutting. This teaching involves providing a suction region under the cut line to draw off the various by-products from the cutting process and is very similar to some of the disclosure of Schlafli. An additional patent which is indicative of the state of the existing art is U.S. Pat. No. 3,569,660 entitled Laser Cutting Apparatus filed on July 29, 1968 by Houldcroft. This apparatus is similar to much of the art discussed above and introduces the concept of providing an assist material which causes a fluxing action in the work area.
A common shortcoming of all this art is the requirement for transmissive optics in the path of the cutting beam if gas is applied to the cut region along the axis of propagation and this in turn limits the amount of power which can be concentrated on the workpiece. Further, the ability to direct a stream of gas through the cut region is limited in each of these devices because the gas being directed into the cut along the axis of propagation subjects the transmissive optics to pressure loadings. Various attempts to improve cutting have been made using a gas jet which is directed onto the workpiece from an off axis location. This type of system obviously is not pressure limited since the gas and the laser radiation travel along different axes of propagation and the transmissive window is not subject to pressure loadings. However, since the gas jet is off axis, not all of the gas momentum serves to remove molten metal and cutting performance is significantly reduced compared with what could be done with a coaxially flowing assist gas.